Field of the Invention
This invention relates to a hydrostatic positive displacement machine with a cylinder drum positioned so that it can rotate around an axis of rotation and which is provided with at least one piston bore in which a piston is positioned so that it can be displaced longitudinally. During the rotation of the cylinder drum, the piston bore is placed in communication alternately with an inlet side and an outlet side. The inlet side and the outlet side comprise connections to a control plate and, located in the reversal area between the connections to the control plate, is a reversal device which damps the pressure adjustment between the displacement chamber and the pressure present in the connection by means of a volume flow into or out of the displacement chamber formed by the piston and the piston bore.
Description of Related Art
On hydrostatic positive displacement machines of this type, when the cylinder drum rotates around the axis of rotation, the piston bores, which form corresponding displacement chambers filled with hydraulic fluid, reverse from the inlet side to the outlet side. The inlet side and the outlet side are in communication via corresponding connections with a control plate with which the cylinder drum is in contact. The piston bores are each provided with a corresponding connection opening, by means of which the piston bore actuates the connections in the control plate.
Because in the operation of the positive displacement machine different pressures are present on the inlet side and the outlet side, when there is a pressure reversal in the piston bore, high volume flows occur when the displacement chamber comes into communication with the corresponding connection on account of the elasticity of the hydraulic fluid and the pressure difference between the displacement chamber and the corresponding connection in the control plate. To prevent high pressure peaks in the communication of the displacement chamber with the connection, it is known that a reversing device can be located in the reversing area between the connections of the control plate, by means of which, when a pressure increase in the displacement chamber is required, hydraulic fluid under pressure is transported to the displacement chamber, or when a pressure decrease in the displacement chamber is required, hydraulic fluid under pressure is removed from the displacement chamber. Reversal devices of this type thereby make it possible, by means of a volume flow into the displacement chamber or out of the displacement chamber, to adjust the pressure between the displacement chamber formed by the piston bore and the connection, so that as a result of the pressure adjustment between the displacement chamber and the pressure that is present in the connection, the existing pressure differences can be damped during the reversal of the displacement chamber into the connection.
A hydrostatic positive displacement machine is described in DE 197 06 114 C2. The reversing device is formed by a hydraulic buffer which comes into communication with the displacement chamber by means of one or more connecting borings which are located on the control plate in the reversing area between the two connections. The connecting borings make it possible to add and remove hydraulic fluid to and from the displacement chamber by means of an appropriately sized cross section that damps the volume flow, so that by means of a damped pressure decrease or pressure increase in the displacement chamber, high pressure peaks can be prevented when the displacement chamber comes into communication with the corresponding connection.
In DE 197 06 114 C2, a plurality of connecting borings in the reversal area—viewed in the direction of movement of the cylinder drum—can be arranged behind one another and therefore sequentially are opened in a chronological sequence one after another during a rotation of the cylinder drum along the reversal area of the connection opening of the piston bore. The plurality of connecting openings makes possible a controlled enlargement of the opening cross section formed by the individual connection openings.
As a rule, the volume flows through these connecting borings of the reversing device are high, such that the pressure of the hydraulic fluid in the damping cross sections of the connecting borings decreases so that cavitation occurs and corresponding cavitation bubbles are formed in the hydraulic fluid. When there is a subsequent pressure increase in the direction of flow of the connecting borings, these cavitation bubbles re-form implosively with a corresponding generation of noise. In the vicinity of material surfaces, for example on the control plate or the cylinder drum, the material surfaces can be damaged by the implosion pressures such that an erosion of material occurs which reduces the service life of the positive displacement machine. The amount of this material erosion caused by the cavitating flows in the connecting borings of the reversing device is proportional to the square of the cross section of the connecting boring and therefore to the fourth power of the diameter of the connecting boring.
Because the cross sectional area of the connecting boring of the reversing device must be designed so that the desired pressure adjustment of the pressure in the displacement chamber is achieved by means of a corresponding volume flow, in known reversing devices with individual connecting borings arranged one behind another in the direction of movement of the cylinder drum, a high degree of material erosion occurs on account of the necessary diameter of the individual connecting borings as a result of the jet cavitation in the connecting borings.